This invention relates to a semiconductor integrated circuit device, such as a flash memory or the like, and to a data processing system, such as a digital still camera, in which such a semiconductor integrated circuit device is employed.
An example of a flash memory device has been disclosed in the 1994 Symposium on VLSI Circuits, Digest of Technical Papers, pp. 61-62.
In this flash memory, a state in which the threshold voltage of each of the memory cells included in the flash memory is high, and a state in which the threshold voltage thereof is low, can be defined as, for example, an erased state and a written (programmed) state, respectively. In this case, writing can be performed after erase operations have been performed collectively in word line units, for example. Upon completion of erase and write operations, the application of pulse-shaped voltages and a verify operation are repeatedly performed until a desired threshold voltage is acquired so that a change in threshold voltage is not increased undesirably.
When the application and transition of the voltage from the threshold voltage in the erased state to the threshold voltage in the written state has been completed, it is difficult to vary the threshold voltage as the threshold voltage approaches the written state. Therefore, the application of the same pulse width will lead to a state in which only the verify operation is being performed even though the threshold voltage changes very little. Therefore, when it is desired to perform writing using a fixed write voltage level, the pulse width is made long as the threshold voltage approaches the written state. The voltage may be gradually increased as an alternative to the gradual increase in pulse width.
High-accuracy writing has heretofore been realized so that a write level (equivalent to a verify word line voltage at writing) is set as, for example, 1.5 V with respect to a power source voltage Vcc of, for example, 3.3 V, and a write pulse or the threshold voltage of each memory cell varies over a range from 0.1 V to 0.2 V.